Update changelog with last minute bugfix

ChangeLog.txt

@@ -11,6 +11,7 @@ Sigil-0.9.11
     - fix bug in closing some tabs needlessly when linking stylesheets
     - fix crash bug in gumbo parser under very strange circumstances
     - remove empty guide elements in epub2 opf as epubcheck now flags these
+    - fix issue with index generation reordering spine entries under certain conditions
 
    New Features
     - update official builds to use Qt 5.11.2 and Python 3.7